Founded in Boston in 1860, the General Theological Library originally lent books by mail to clergy living in remote areas. In 1988, a group of Maine clergy and laypersons negotiated a deal to move the collection to its present location in Portland's historic State Street Church. A few years later, when the seminary opened a campus in Portland, the two organizations merged, and the collection of the General Theological Library became part of the Seminary's second library.

The collection includes approximately 35,000 volumes. Students in Portland also have full access to the holdings of Moulton Library. Requests for material from Moulton Library should be submitted to the Assistant Librarian in Portland.